Computational and experimental study on structure-photoactivity relationships of formate esters and perfluorinated anhydrides

Abstract

Structure-activity relationship study is an important research field in chemistry, biology and pharmacology. The structure-photoactivity relationship study reported here focuses on the effect of molecular structure on photochemical reactivity of formate esters and perfluorinated anhydrides. The Restricted Hartree Fork (RHF) method was used for tert -butyl formate ground states at 3-21G, 6-31G and 6-31G* levels, and the Unrestricted Hartree Fork (UBF) method was performed for the first triplet excited state, and the triplet 1,4-diradicals of tertbutyl formate at 3-21G* level. The MP2/6-31G* single point energy calculations were performed on RHF/6-31G* or UBF/3-21G* optimized structures. Two stable conformers were located in ground state, s-cis conformer and s-trans. The s-cis conformer was found to be more stable than s-trans conformer by 5.0 kcal/mol and the internal rotation barrier around C-O ester bond was 11.4 kcal/mol. Three different precursors, perfluoroacetic anhydride (PFAA), perfluoroglutaric anhydride (PFGA) and perfluorosuccinic anhydride (PFSA), were chosen for comparison of CO2 bending excitation. The total available energy, estimated by using the energy/group contribution method of Joshi, varies from approximately 29 kcal/mol for PFAA to 95 kcal/mol for PFSA. Flowing 100 mtorr samples of perfluorinated anhydrides were photolyzed with an exciter laser operated at 222 nm. Time-resolved IR diode laser absorption spectroscopy was used to determine the relative populations of a variety of CO2 rovibrational states. The photodissociation dynamics of gas-phase PFAA at 240 Mn was studied by using time-resolved IR diode laser spectroscopy and ab initio calculation with 3-21G* basis set. The CO photoproduct was found only in v = 0 state. IR absorptions were measured at 011 0, 02 2 0 and 033 0 and the bending vibrational states of CO 2 . The bending vibrational temperature determined by the populations at 011 0, 022 0 and 033 0 states is 1300 ± 400 K under 240 mn excitation while the temperature determined by the populations at 011 0, 022 0 and 03 3 0 states is around 1100 ± 20 K under 222 nm light. The average bending vibrational energy of CO2 derived from experimentally observed mode temperature is 1650 ± 60 cm-1 with 222 nm photodissociation and 1900 ± 500 cm-1 with 240 nm photodissociation. The average vibrational energy of CO is negligible with 240 nm photodissociation. These results demonstrate the power of combining diode IR laser absorption spectroscopy technique with ab initio calculation method for the photodissociation dynamics of polyatomic molecules. (Abstract shortened by UMI.)